The ancestors of present-day odontocetes probably evolved during the Oligocene Epoch (33.7 million to 23.8 million years ago) from a group of more ancient whales called archaeocetes, which also had teeth, as did some early baleen whales. Some odontocetes have only vestigial teeth buried in their jawbones; others have teeth that are erupted, in numbers varying from 1 in the narwhal to more than 240 in the franciscana, or La Plata river dolphin. The teeth are simple cones that function only for grasping, not chewing. The teeth are also used in aggressive behaviour among members of the same species to seize or shake an opponent.

Odontoceti is a term derived from the Greek odontos (“tooth”) and ketos (“whale”).